One of the methods for diagnosing the degree of degradation of a concrete structure, such as a bridge, a tunnel, and a building, includes a measurement of widths of cracks (i.e., dimensions in a direction perpendicular to a crack progressing direction) occurred on the surface of concrete. The typical method for measuring the crack widths uses a conventional scale or a crack scale exclusively designed for crack measurement which is placed on the concrete surface to visually measure the crack width. Disadvantageously, this measurement limits the measuring places within a reach of the measurer.
JP 08-947752 A proposes a crack measuring device Which includes an elongate rod and a scale or a crack scale attached on the tip of the rod. This also limits the measuring places within a range of the rod, so that the cracks on the bridge beam or the top ceiling of the tunnel are incapable of being measured even by this device.
Accordingly, there is no option other than to use a vehicle designed exclusively for works at elevated places or a ladder for measuring cracks occurred on higher portions of the concrete structure, which requires much time for the crack measurement. Still, the cracks on higher places outside the reach of the vehicle or the ladder are unable to be measured.
Therefore, the present invention provides an optical apparatus which allows for measuring dimensions of objects such as cracks of about 0.1 millimeter to several millimeters width and occurred on concrete structures, for example, from a remote place (e.g., a place several to several hundreds of meters away from the objects) and a method for measuring the dimensions of the objects by using the optical apparatus.